*{
    margin:0;
    padding:0;
}
nav{
    width: 750px;
    height: 320px;
    margin:20px auto;
    overflow: hidden;
    position: relative;
}
#index{
    position: absolute;
    left:320px;
    bottom: 20px;

}
#index li{
    width:8px;
    height: 8px;
    border: solid 1px gray;
    border-radius: 100%;
    background-color: #eee;
    display: inline-block;
}
#img{
    width: 750px;/*不给宽高无法移动*/
    height: 320px;
    position: absolute;/*不定义absolute，js无法设置left和top*/
    z-index: -1;
}
#img li{
    width: 750px;
    height: 320px;
    float: left;
}
#index .on{
    background-color: black;
}